To illustrate a conventional handover between a first and a second technology network the handoff scenario depicted in FIG. 1 is considered. FIG. 1 shows a footprint of a first technology network, e.g. WLAN (Wireless Local Access Network), in a hot spot, which in this case is an indoor environment such as a hotel. The bright circles indicate strong signal footprints of different WLAN access points APs located at the hot spot, while the gray circle indicates a strong signal footprint of an access node of a second technology network, e.g. a cellular base station. As it is shown in FIG. 1, the WLAN geographical coverage area is subsumed in a cellular geographical coverage area, as typically is the case in practice.
It is supposed that a mobile user having multi-radio (WLAN and cellular) mobile terminal initiates an Internet session over WLAN access and walks along the path shown in FIG. 1 by dotted line. Furthermore, it is supposed that the user has chosen WLAN as the preferred access to the IP network. This is mainly because WLAN access it typically cheaper and faster than cellular access. However, the user also wants the connectivity to be seamlessly handed over to the cellular network when WLAN coverage is not available anymore.
According to the conventional technology, initiating such inter-technology handoff mainly is based on a signal strength criterion. This has disadvantages such as causing spurious handoff triggers when the user is still within the WLAN footprint but looses connectivity due to short gaps in the coverage as can be seen from FIG. 1. This is because at such gaps the mobile terminal might sense a diminishing WLAN signal but a strong cellular signal.
For example, along his path shown in FIG. 1, when the user comes in an area where WLAN signal strength drops due to local factors such as metal objects, walls etc., the mobile terminal detects a drop in WLAN signal strength, but a strong cellular signal. Based on the signal strength criterion alone, the mobile terminal will immediately try to initiate handoff from WLAN to cellular network. However, moments later the mobile terminal again detects strong signal from WLAN, and hence, initiates handoff back to WLAN network. Such a ping-pong effect is undesirable as it causes unnecessary signaling traffic as well as may cause more disruption in service than what would have been caused due to lack of WLAN signal for a short while.
Of course, the mobile terminal could wait for a short interval of time after the loss of WLAN signal before initiating handoff to the cellular network. However, this degrades the (latency) performance of handoff when the user really exits the hotel door, and hence, loses WLAN connectivity for real.
A further problem encountered with the prior art mainly relying on the signal strength criterion is the difficulty in implementing proactive procedures of handoff.
The performance of inter-technology handoff (or IP-layer handoff in general) can be improved if some of the handoff messaging can be done proactively, i.e., while the mobile terminal still sees a strong WLAN signal. These proactive steps could include, for example, acquiring the cellular network, performing Mobile IP Fast Handoff signaling as described in “Low Latency Handoffs in Mobile IPv4”, Internet draft, draft-ieff-mobileip-lowlatency-handoffs-v4-04.txt, June 2002, and “Fast Handovers for Mobile IPv6”, Internet draft, draft-ietf-mobileip-fast-mipv6-05.txt, September 2002, authenticating with the cellular network, performing Candidate Access Router Discovery as described in “Issues in Candidate Access Router Discovery for Seamless IP layer Handoffs”, Internet draft, draft-ieff-seamoby-cardiscovery-issues-04.txt, October 2002, informing the target network about QoS (Quality of Service) and other requirements of current applications, etc. Then, as soon as the WLAN signal strength diminishes, the mobile terminal can send a final trigger to the cellular network to complete the handoff process (i.e., actually commit the radio resources, arrange for packet rerouting, etc.).
However, the need for handoff has to be anticipated with enough margin for the execution of proactive handoff steps. It is not possible to do this reliably based on signal strength measurements alone. For example, if the proactive handoff procedures are started after WLAN signal strength starts to decline noticeably, the Internet connectivity of the mobile terminal may disappear before connectivity can be established with the cellular network. On the other hand, if handoff procedures are started too early (when WLAN signal is still strong), it may result in large number of false starts. In other words, the mobile user may never actually leave the WLAN coverage (e.g. he is sitting in a hotel room when call is initiated over WLAN), even though proactive handoff steps have been started. This may create unnecessary signaling burden on the network.
According to the WO 99/48320, certain gateway cells are used for controlling handovers in and outside a building for controlling pico/macro cell handovers. A gateway cell contains information on neighboring cells in it's neighbor list. However, the WO 99/48320 requires that the networks are controlled by the same operator.
Moreover, in the WO 96/31078, a method is disclosed which is used to indicate a border crossing which is based on mobile node listening to a pilot signal from outside the border, informing about that to a BSC (Base Station Controller), and indicating the border crossing from the BSC to the mobile node.